1. Field of the Invention
This invention relates generally to the field of wireless data communications and more particularly to a wireless data communication network utilizing frequency hopping at radio frequencies.
2. Background of the Invention
Many possibilities exist for wireless data communication systems operating at various frequencies. At the low end of the frequency spectrum audio, ultrasonic and carrier current systems can provide safe low cost data communications. Unfortunately they suffer from limited data capacity and in some instances marginal performance. Higher frequency systems utilizing microwave to infrared and laser offer extremely large potential data capabilities. Unfortunately such high frequency systems which exhibit reasonable range, tend to be expensive and point to point in nature. They may also represent health hazards in some situations. A high frequency system that is dispersive enough not to be an eye hazard may be limited in range to the point that a wide degree of flexibility is unachievable. Also, such systems frequently suffer serious performance degradation in the presence of dust, rain, smoke or smog.
The intermediate spectrum of the radio frequency range offers a reasonable compromise between acoustic and optical systems in many instances. The drawback to this equipment is that it most frequently requires licensing from the Federal Communications Commission (FCC). This drawback can be avoided if the system is operated within the constraints defined presently by 47 CFR, Part 15, Section 122 which outlines the FCC Rules and Regulations concerning such systems.
These regulations allow for unlicensed operation in the frequency ranges of 40.66 to 40.70 megahertz and 70.00 megahertz and above providing that (among other things) each transmission is limited in duration to less than one second out of every 30 seconds and a duration of at least ten seconds between transmission at a particular frequency. It is therefore possible by adhering to these guidelines to produce a wireless data communications system which does not require FCC licensing and has appreciable data throughput.
While at first it appears that a simple frequency hopping transceiver system can be readily designed to meet the above limitations, a number of practical problems are encountered when attempting to design such a system. A careful examination of the regulations indicates that it is desirable to hop among at least 30 channels. To provide a reasonably high data rate the channels should be separated by at least 10 khz but this is not to be limiting. It is also desirable to utilize approximately the full one second allowed by the regulations on each individual frequency to minimize time wasted during hops. It is also desirable to provide such a system at a minimal cost and complexity.
In developing a system to meet these goals, most designs would invariably require rather large frequency changes at some point in time. In synthesizer designs, this presents a problem in acquiring fast lock in a phase locked loop synthesizer or rapid acquisition of the transmitted signal at the receiver when frequency changes are made. To avoid such problems, it is desirable to minimize the distance in frequency between frequency steps where possible. The present invention is directed toward this problem in particular.
The present invention effectively and inexpensively avoids this problem of attaining rapid frequency stability while adhering to the FCC Regulations so that maximal data throughput can be achieved without wasting valuable transmission time in allowing frequency synthesizers to settle or assuring that receiving have achieved lock.